Security is resilience with respect to your threat model. The usual security objectives are confidentiality, integrity, and availability.

Nothing is "secure" forever and ever. For each objective you need to state some assumptions that are required to uphold it.

Haskell allows you to express more assumptions in code, not just comments, to be enforced at compile time. And with some careful handling Haskell types provide a solid foundation without going into full deductive verification mode.

One example of security is not munging strings together and shipping it off to the database or some other "eval" with full privileges. A programming language with lightweight static types makes it easier to parse outside data into a typed representation, then transform/evaluate/compile as necessary. This is of course Haskell's bread and butter.